Semiconductor laser arrays have recently attained optical power outputs which make them attractive for many high power applications, such as for pumping solid state lasers. For example, AlGaAs buried heterostructure lasers produced with silicon impurity induced disordering (IID) have achieved low threshold, high efficiencies, and power levels of up to 0.5 W cw. One limitation, however, on output power capability for AlGaAs lasers is catastrophic damage to the laser facets or mirrors, due in part to local heating at high output powers from optical absorption in the active region near the facets. Additionally, at somewhat lower power levels, facet erosion caused by oxidation of the active region may occur thereby reducing the useful life of the laser. It is known that improved catastrophic damage levels can be obtained either by shifting the laser emission to a longer wavelength, i.e. lower energy, relative to the threshold absorption energy at the facet or by increasing the bandgap of the facet material, i.e. the absorption energy at the facet, relative to the laser emission energy. In either instance, regions adjacent to the facets characterized by substantially lower optical absorption are created. These regions are called "windows" and lasers having these facet windows are called "window lasers".
Window lasers have been fabricated by a variety of methods. One method is to introduce a change of material composition by the selective diffusion of zinc into the laser cavity everywhere except at the facets while maintaining a constant thickness waveguide layer. The diffused zinc shifts the laser emission to a longer wavelength. The power output of zinc diffused window lasers is limited by catastrophic damage due to local heating in the bulk rather than at the facets.
More recently various methods have been used to increase the effective bandgap at the facet, including zinc impurity induced disordering and etching and regrowth. Blauvelt et al. in Applied Physics Letters, vol. 40, no. 12, June 15, 1982, pp. 1029-1031, describe a buried heterostructure window laser produced by the latter method. Portions of the active layer which will eventually form the window regions are removed by selective etching, then a thicker transparent waveguide or window of wider bandgap material is regrown in its place.
Another method for increasing the available power level limited by catastrophic facet damage is to grow very thin active layers to lower the optical power density at the facets. In order to avoid increasing the threshold current because of a decrease of gain, nonuniformity of the active layer for liquid phase epitaxy grown layers, and the influence of the active layer interfaces, the active layer is tapered, i.e. made thinner only near the facets. Burnham et al. in U.S. Pat. No. 4,546,480 and Murumaki et al. in Electronics Letters, vol. 22, no. 4, Feb. 13, 1986, pp. 217-218, disclose two distinct lasers produced by this technique.
It is desirable, when producing window regions in lasers, not to significantly increase scattering, propagation and diffraction losses. In order to reduce diffraction loss in the vertical direction perpendicular to the plane of the active region, it is preferred that a waveguide remain in the window regions to confine the light. Unfortunately, some window formation techniques, such as impurity induced disordering, may completely destroy the vertical waveguides in the window region. Other window formation techniques, such as etching and regrowth, can introduce abrupt boundaries that cause large scattering losses. Thinning the active region can cause propagation losses which depend on the degree of thinning and the length of the window regions.
An object of the present invention is to produce a semiconductor window laser with high power output having reduced far field divergence, and without substantially increased losses in the laser cavity.